Waterless plates have been fabricated since 1970. Most current manufacturing procedures employ one or more of the following three imaging techniques:
1. A light-sensitive layer underlies the repellent silicone layer, remaining there after development in the exposed areas that become the non-image
2. The repellent silicone layer is directly attached to the support, with none of it removed in development; the image is made by attaching ink-receptive particles to the silicone, with these particles becoming the image
3. The repellent silicone layer is directly attached to the support, with none removed during development; the image is formed by altering the silicone layer by utilizing electrical discharge techniques
There are many variations on the above techniques, as befitting this creative and imaginative art. In some cases, several other materials are introduced between the repellent silicone layer and the support.
The ability to produce a high-quality waterless planographic plate has heretofore proved elusive. One troublesome problem is that it is difficult for silicone coatings to adhere to organic materials. Therefore, when such materials underlay the silicone coating, poor adhesion results. Another critical situation is created when high heat over long cure cycles deteriorates organic materials, especially photosensitive ones. This makes heat post curing of the silicone layer difficult or even impossible. Another dilemma plaguing the industry is posed by the plates being easily degraded and destroyed. Still another deficiency of the existing plates is their inability to be imaged by any method other than photographic methods. This makes corrections difficult and the use of direct imaging (such as laser printing or xerography) impossible.
The present invention reflects the discovery that low, free-surface energy polymers chemically bonded to the substrate produce suitably repellent areas for the non-print zones of waterless plates. Dimethylpolysiloxanes have such properties. They can be made into tough, elastic films by catalytic reaction, after which they also possess certain reactive groups. For purposes of this invention, the film must be adherent to the non-print plate surface, usually aluminum oxide. The low, free-surface energy polymers must also be sufficiently permeable to allow a developer to pass through to the image. In turn, the developer must be capable of producing a precise image. The word "precise" in this case indicates that the demarcation between the image areas and the non-print zones must be preserved exactly as they were on the original plate.
Dimethylpolysiloxanes with terminal hydroxyl sites are suitable prepolymers, but this inventor has found that they will only provide precise, well-adherent images when they are mixed and cross-linked with dimethylpolysiloxanes having certain other, reactive terminal groups. The reactive terminal groups allow for a strong cross-linking of the polymers, so that the development of the non-print areas will be exactly preserved. A chemical linkage occurs between the polymer silane groups and the metal oxide of the substrate, resulting in a very adherent film.